This invention is directed to a new platelet form of 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]pyrrole and to a method of formation of the new platelet form of 1,4-diketo-3,6-diphenylpyrrolo[3,4-C]-pyrrole. The new platelet form is prepared by heating 1,4-diketo-3.6-diphenylpyrrolo[3,4-C]-pyrrole to elevated temperatures and can be made from crude 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]-pyrrol. The new platelet form of 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]-pyrrole demonstrates a unique color shade effect and excellent heat resistance when utilized as a pigment.
The use of 1,4-diketo-diaryl-pyrrolo-[3,4-C]-pyrroles as pigments is well known in the art, for example as disclosed in U.S. Pat. No. 4,415,685. These compounds have generally been found to be pigments of excellent quality.
In addition, many patents describe after-treatment, or so-called "conditioning" processes, to improve the quality of 1,4-diketo-diaryl-pyrrolo-[3,4-C]-pyrrole pigments.
U.S. Pat. No. 4,579,949 provides a procedure for obtaining a more opaque pigment form by heating 1,4-diketo-diaryl-pyrrolo-[3,4-C]-pyrrole pigments in water or an organic solvent. U.S. Pat. No. 4,720,305 teaches heat treatment of a 1,4-diketo-diaryl-pyrrolo-[3,4-C]-pyrrole pigment-water or organic solvent mixture (preferably above 80.degree. C.) and subsequent comminution, such as wet grinding, in order to prepare transparent pigment forms.
U.S. Pat. No. 4,992,101 describes a process for increasing the opacity of 1,4-diketo-diaryl-pyrrolo-[3,4-C]-pyrrole pigments by milling the pigment in an alcohol in the presence of a base at a temperature below 50.degree. C.
U.S. Ser. No. 07/726,612 discloses a process for converting crude pigments into pigmentary form by first premilling the pigment crude and then contacting the premilled pigment with a polar solvent at temperatures below 50.degree. C., with or without particle growth inhibitors.
The 1,4-diketo-diaryl-pyrrolo-[3,4-C]-pyrroles described hereinabove are high chroma pigments with excellent pigment properties but do not offer lustre effects. To overcome this drawback, U.S. Pat. No. 5,095,122 discloses a 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]-pyrrole organic pigment which is in platelet-shape form and so achieves the desirable pearl lustre pigment without resorting to additives. The 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]-pyrrole consists of 50% by weight of platelets which are from 5 to 50 .mu.m long, 2 to 50 .mu.m wide and 0.01 to 5 .mu.m thick, with the proviso that the ratios of length:thickness and width:thickness must be at least 3. The pigment platelets are formed by recrystallization in an organic solvent in which the pigment is soluble to a certain degree.
The platelets of the present invention are distinguished from those of U.S. Pat. No. 5,095,122 by the X-ray diffraction pattern, which clearly demonstrates that a different pigment form is produced under the present recrystallization conditions. The present process of preparing the crystals is distinguished from that disclosed in U.S. Pat. No. 5,095,122 by requiring that the 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]-pyrrole be heated to higher temperatures than those disclosed in U.S. Pat. No. 5,095,122. The new platelet pigment form of 1,4-diketo-3,6-diphenylpyrrolo-[3,4-C]-pyrrole has improved heat stability when incorporated in high-performance plastics, and a particle size which is preferred for use as a stir-in pigment for automotive coatings.